The present invention relates to methods and apparatus for sputter coating articles, and especially, for reactive sputter coating of plastic ophthalmic lens elements using a sputter source with multiple anodes. As used herein, lens elements include, according to context, edged lenses, semi-finished lenses and lens blanks. Also included are wafers for forming laminate lenses or wafer blanks therefor. Ophthalmic uses of the lens elements include uses in eyeglasses, goggles and sunglasses.
Many ophthalmic lenses produced today are made from a single plastic body or laminated plastic wafers. The plastic material may include thermoplastic material such as polycarbonate or thermoset material such as diallyl glycol carbonate types, e.g. CR-39 (PPG Industries). The material may also be a cross linkable polymeric casting composition such as described in U.S. Pat. No. 5,502,139 to Toh et al. and assigned to applicant.
Ophthalmic lens elements are frequently coated to achieve special properties. Anti-reflection coatings improve the transmittance of visible light and the cosmetic appearance of the lenses. Reflective and absorptive coatings may be employed in sun lenses to reduce light transmittance to the eye, to protect the eye from UV radiation and/or to impart cosmetic colorations to the lens. Coatings may also provide other beneficial properties such as increased hardness and scratch resistance and anti-static properties.
Desirable lens coatings may be created by applying single or multiple layers of metal, metal oxides or semi-metal oxides to surfaces of the lens element. Such materials include oxides of silicon, zirconium, titanium, niobium and tantalum. Metal and semi-metal nitrides are also used. Examples of such multilayer coatings are given, for example, in U.S. Pat. No. 5,719,705 to Machol entitled xe2x80x9cAnti-static Anti-reflection Coatingsxe2x80x9d, assigned to applicant. Interference filter coatings for sunglasses are disclosed, for example, in U.S. Pat. No. 2,758,510 to Auwxc3xa4rter. Other lens coatings are disclosed in International Application WO 99/21048 to Yip, et al., which is hereby incorporated by reference.
Various methods are disclosed in the prior art for applying metal and semi-metal oxide coatings to ophthalmic lenses. Such coatings have traditionally been deposited by means of thermal evaporation, and more recently, by electron-beam (e-beam) evaporation and reactive sputtering. Evaporations are typically carried out at vacuums better than 10E-5 Torr. Ritter et al. U.S. Pat. No. 4,172,156, for example, discloses e-beam evaporation in an oxygen atmosphere of Cr and Si to form coating layers on a plastic lens. The use of reactive sputter deposition to form various oxide layers on lens elements is disclosed in the above-mentioned ""705 patent to Machol.
Reactive sputtering in general is a conventional technique often used, for example, in providing thin oxide coatings for such items as semiconductor wafers or glass lamp reflectors. Examples of various conventional vacuum deposition systems for the formation of coatings by reactive sputtering are disclosed in the following patents: U.S. Pat. No. 5,616,224 to Boling; U.S. Pat. No. 4,851,095 to Scobey et al.; U.S. Pat. No. 4,591,418 to Snyder; U.S. Pat. No. 4,420,385 to Hartsough; British Patent Application GB 2,180,262 to Wort et al.; Japanese Kokai No. 62-284076 to Ito; and German Patent No. 123,714 to Heisig et al.
The coating of plastic lenses in spinning drum coaters by means of sputtering technology, including DC reactive sputtering, is a relatively recent development. A conventional drum vacuum coating system used for this purpose is shown in FIG. 1. The system includes a vacuum chamber 11, which contains a hollow workpiece holder or drum 12. Lens elements, such as lens 13 are arranged in columns on an external surface of the drum 12. A coating applicator 14 is located near a wall of the vacuum chamber adjacent the drum 12. The coating applicator 14 may comprise a combination of magnetron sputtering sources and microwave plasma generators with a reactive gas supply such as disclosed in U.S. Pat. No. 5,616,224 to Boling, which is hereby incorporated by reference. Power is delivered to the coating applicator 14 by one or more power supplies (not shown) via an electrical lead assembly 17. A reversing power supply for arc suppression such as disclosed in U.S. Pat. No. 5,616,224 to Boling may be included. A sputtering gas is introduced into the vacuum chamber through gas-supply plumbing built into the coating applicator or through a separate port (not shown) on the vacuum chamber 11. The sputtering gas is controlled by a gas controller (not shown) and may be an inert gas such as argon or a reactive gas mixture such as argon/oxygen or argon/nitrogen.
The vacuum chamber 11 is evacuated by vacuum pumps (not shown) attached to a pumping plenum 15. A cryopumping surface, known as a Meissner trap, is conventionally provided in the form of cryocoils 16 in the plenum 15. A coolant with a temperature well below the freezing point of water flows through the cryocoils 16, allowing the Meissner trap to remove water vapor from the vacuum chamber 11. A Meissner trap may be advantageously configured in the vacuum chamber 11 rather than in the pumping plenum 15 to improve the cryopumping of water vapor. Such a configuration is especially useful when coating plastic lens elements because plastic lens elements have a tendency to outgas substantially more water vapor than conventional glass lenses, as disclosed in U.S. Pat. No. 6,258,218, hereby incorporated by reference.
A drum vacuum coating system with an elongated magnetron sputter source 14 such as that illustrated in FIG. 1 provides a convenient means of coating numerous lens elements or other articles. However, Applicants have observed that such systems typically do not produce uniformly thick coatings on multiple articles disposed in a given column of the drum 12 due to the variation in sputter rate along the length of the sputter source. In other words, an article or lens element positioned near the top of a given column may not receive a coating of the same thickness as an article or lens element positioned near the center of that column.
Several methods directed toward improving coating uniformity of sputtered films have been disclosed in the prior art. U.S. Pat. No. 5,645,699 issued to Sieck discloses a system comprising two cylindrical magnetron sputter sources, each with an anode substantially spanning the length of the sputter source, wherein the placement of a third anode between the two sputter sources has improved coating uniformity. U.S. Pat. No. 4,849,087 issued to Meyer discloses the use of multiple gas nozzles distributed along the length of a sputter source to deliver varying amounts of an argon/oxygen gas mixture to local regions of the plasma above the sputter target (cathode). Individual resistance probes disposed along the width of the substrate measure the local resistance of the coating and provide feedback signals to adjust the gas flow through the various nozzles to maintain uniform electrical resistance in various regions of the coating. While this approach provides control of the electrical resistance of the coating, it does not necessarily provide control of the coating thickness, a quantity of importance for optical coatings.
U.S. Pat. Nos. 5,487,821 and 5,683,558 to Sieck et al. disclose the use of xe2x80x9cwire brushxe2x80x9d anodes in conjunction with magnetron sputter sources and indicate that the wire-brush point density of an anode may be adjusted along the length of the sputter source to affect the uniformity of the deposited film. U.S. Pat. No. 5,616,225 issued to Sieck et al. discloses the use of wire brush anodes and the use of multiple anodes in conjunction with a single magnetron sputter cathode for coating substrates (especially large substrates) wherein the anode voltages may be individually controlled. The ""225 patent indicates that this control may be utilized to improve the thickness uniformity of the deposited coating. The disclosure in the ""225 patent, however, does not address controlling the thickness uniformity of reactive coatings deposited on large numbers of individual lens elements using an elongated magnetron sputter source in a drum vacuum coating system.
A need still exists to provide drum vacuum deposition systems for high volume production of individual articles, such as plastic lens elements, while ensuring a high degree of control over the thickness and composition of the coatings.
Accordingly, it is an object of the present invention to improve the degree of control over the thickness and composition of thin metal and semi-metal oxide coatings deposited on multiple articles, particularly plastic lenses, disposed on a rotatable holder in a vacuum coating system.
It is another object of the present invention to provide a multi-anode sputter source adapted to the geometry of a cylindrical drum vacuum coating system for depositing coatings on numerous plastic parts.
It is another object of the present invention to provide an apparatus for depositing a high quality coating on large numbers of plastic lens elements in a system which is relatively inexpensive to construct and operate.
These and other objects and features of the present invention will be apparent from the written description and drawings presented herein.
A drum vacuum coating system with an elongated magnetron sputter source provides a convenient means of coating numerous lens elements or other articles located on a rotatable drum. However, Applicants have observed that such systems typically suffer from the inability to produce coatings of uniform thickness on multiple articles disposed in various locations on the drum due to variations in the sputter rate along the length of the sputter source. Applicants have determined that providing an elongated sputter source with multiple anodes, wherein the currents to the anodes may be individually controlled or controlled in pairs, allows the deposition of coatings of substantially uniform thickness on multiple articles regardless of their position on the drum. The thickness uniformity is acceptable for thin optical coatings on ophthalmic lens elements.
A preferred embodiment of the present invention is a method and apparatus for sputter coating a plurality of articles such as plastic lens elements. The system includes a vacuum chamber, a rotatable cylindrical holder for holding the plurality of articles, and at least one sputter source that is elongated along a lengthwise direction and that has a cathode and a plurality of anodes. The sputter source is disposed with its lengthwise direction parallel to a rotation axis of the cylindrical holder, and the anodes are disposed adjacent the cathode substantially along at least one line parallel to the lengthwise direction. The articles may be disposed in a predetermined pattern on the cylindrical holder, and the anodes may be disposed in positions corresponding to positions of the articles disposed on the cylindrical drum. Additionally, the articles may be arranged in columns and rows on the cylindrical holder, the columns being parallel to the rotation axis of the cylindrical holder, and the anodes may be configured in pairs, each anode pair being aligned with a row on the cylindrical holder. The sputter source may be a planar magnetron sputter source.
A cathode power supply provides a negative voltage to the cathode, and a separate anode power supply with a plurality of channels provides anode currents to the anodes. Alternatively, the cathode power supply and the anode power supply may be provided in a single unit. The anodes may be configured in pairs, and each anode pair may be powered by a separate channel. In addition, the anode currents may be adjusted in a manner to produce coatings of increased uniformity of thickness on articles positioned in different locations on the cylindrical holder. In one embodiment, the same amount of current is provided to each active anode pair by the controlled power supply.
The length of the sputter source may range from twenty inches to sixty inches, though approximately forty inches is a preferred length. The number of pairs of anodes may range from six pairs to fifteen pairs. Eight or nine pairs of anodes are preferred. In addition, it is preferred that the length of each anode is approximately the same as the diameter (height) of the surface of article to be coated. The height and diameter of the drum may be approximately forty inches, and the drum may hold approximately 200 to 400 articles.
In order to prevent the buildup of dielectric material on the electrically active anodes each anode of the sputter source may be configured as an electrically conducting bar having a recessed slot, the slot being oriented such that the opening of the slot is directed away from the cathode. In this manner, anodes are provided with interior surfaces that remain substantially free of dielectric coatings, ensuring good electrical conduction between the anodes and the sputter plasma. Alternatively, the anodes may be configured as wire-brush anodes, wherein each anode comprises a plurality of electrically conducting wires emanating from an electrically conducting support member. This configuration similarly provides anode surfaces that remain substantially free of dielectric coating deposits at the root of the brush.
In another embodiment, the apparatus may be used to carry out reactive DC sputtering of a thin coating, which may comprise a dielectric layer deposited onto surfaces of plural articles such as plastic lens elements. In this case, the apparatus again includes a vacuum chamber and a sputtering source that is elongated in a lengthwise direction and that has a cathode and a plurality of anodes, the anodes being arranged in pairs. The apparatus also includes a rotatable article holder located in the vacuum chamber that rotates the plural articles past the sputtering source, the articles being arranged in a predetermined pattern on the article holder. The article holder may be a hollow drum rotated about its central axis. In addition, the apparatus includes a source of reactive gas, such as oxygen or nitrogen. An elongated plasma applicator, such as a microwave plasma generator, is also provided adjacent to the sputtering source for producing a plasma to facilitate the reaction of the reactive gas with material sputtered from the sputtering source. The sputter source may be a planar magnetron sputter source.
The articles may be arranged in columns and rows on the article holder, each anode pair being aligned with a row on the cylindrical holder. The reactive sputtering may comprise the sputtering of a metal or semi-metal utilizing a sputtering gas that contains oxygen in order to produce metal-oxide or semi-metal-oxide coatings. A sputtering gas comprising nitrogen may also be used to produce nitride coatings. In addition, a second sputtering source may be provided adjacent to the first sputtering source or adjacent to a plasma applicator for sputtering a metal or semi-metal different than that sputtered from the first sputtering source. In this manner, multiple coatings with different indexes of refraction may be provided on articles such as plastic lens elements.
The apparatus may further comprise a controller that receives an input signal corresponding to a small number of measurable process variables (for example, two) and which controls a small number of process variables (for example, two) in response thereto. In a preferred embodiment, the measurable process variables are cathode voltage and total gas pressure; the controlled variables are a first flow rate for a first gas and a second flow rate for a second gas. A purpose of the controller is to maintain batch-to-batch uniformity of coating thickness and coating composition. The controller may be used in roll coating or the coating of discrete articles such as lens elements as described below.
Batch to batch (run to run) stability of deposition rates of the sputtered materials may vary due to a number of causes even when the sputter plasma is held at a constant power dissipation. These causes includes: historical effects on the target (such as oxide coverage and oxygen implantation), target cleaning, tooling cleaning, chamber cleaning, length of time the chamber is opened for loading, unloading or servicing and consequent coverage of chamber and tooling with absorbed layers of water vapor, thickness of deposited materials on tooling and chamber, type of plastic constituting the lens substrates and their degree of water uptake, gas leaks, improper calibration of partial pressure gauges and/or other means of measuring partial pressures such as Optical Gas Controllers. Applicants have determined that, at constant sputter plasma power or constant sputter cathode current, more stable deposition rates may be obtained by manually adjusting two directly controllable process variables (in a preferred embodiment, the flow rates of the buffer gas (usually Argon) and the reactive gas (usually oxygen)) according to a set of rules based on observations of two measured input parameters (in a preferred embodiment, cathode voltage and total pressure). The rules may be experiential rules based on an expert""s understanding of the operation of a particular sputtering system. These rules may be embedded in the fuzzy logic control system. For example input parameters may include classifying three levels of cathode voltage and total pressure: xe2x80x9cLOWxe2x80x9d, xe2x80x9cOKxe2x80x9d and xe2x80x9cHIGHxe2x80x9d. The output parameters for adjusting the operation of the system may include three classifications for the flow rates of the buffer gas and the flow rate of the reactive gas, namely: xe2x80x9cINCREASExe2x80x9d, xe2x80x9cHOLDxe2x80x9d and xe2x80x9cDECREASExe2x80x9d. The fuzzy logic determination may be implemented in control signals for opening or closing buffer gas and reactive gas valves by a precise amount.
In another embodiment, a multi-anode device for use in sputter deposition is provided. Note that the multi-anode device may be used separately from the above described gas control techniques. However, both techniques may be advantageously used to achieve a high degree of coating thickness uniformity.
The multi-anode device comprises a cathode and a plurality of anodes located in predetermined positions adjacent to first and second opposing sides of the cathode. The anodes disposed at each of the first and second opposing sides of the cathode are configured in a regular arrangement to substantially minimize or eliminate gaps between adjacent anodes. Further, a different current may be applied to each anode. Such a configuration may be desirable for facilitating uniformity of the sputter plasma. In particular, the anodes at a given side of the cathode may be configured in a sawtooth arrangement, wherein an end of one anode is disposed closer to the cathode than an end of the adjacent anode. The anodes at opposing sides of the cathode may be arranged identically or in a mirror image arrangement. The anodes may also be arranged in an essentially linear fashion wherein the ends of the anodes approach each other closely but are shielded from one another by interposing an electrical and/or physical barrier between said ends. In another embodiment, the ends of adjacent anodes may be purposely separated from, for example, 1 to 5 inches to modify electrical coupling of the anodes with one another through the plasma.
In addition, each anode of the multi-anode device may be configured as an electrically conducting bar having a recessed slot, the slot being oriented such that the opening of the slot is directed away from the cathode. Alternatively, the anodes may be configured as wire-brush anodes, wherein each anode comprises a plurality of electrically conducting wires emanating from an electrically conducting support member.
In another embodiment, a method for sputter coating a plurality of articles is provided. A vacuum chamber is provided having at least one sputter source with a plurality of anodes configured in pairs and having an article holder that rotates about an axis. Plural articles are located in a predetermined pattern on a radially outward facing surface of the article holder, and the chamber is pumped down. A sputtering gas of the desired composition, flow and pressure is provided, and the article holder is rotated relative to the sputter source. Sputter coating is carried out on the radially outward facing surfaces of the articles while controlling current to each pair of anodes separately.
The articles may be arranged in columns and rows on the outward facing surface of the article holder, and the rows may be aligned with pairs of anodes. The drum may be continuously or sequentially rotated relative to the sputter source or sputter sources while voltages are applied to the cathode and anodes to cause material to be sputtered from the sputtering target onto the radially outward facing surfaces of the articles. Further, the currents to the anodes may be adjusted in a manner to provide a uniform coating to the articles at all positions on the article holder. In addition, the sputter coating may comprise a reactive DC process in which sputtered material reacts with reactant gas to form a dielectric layer. The dielectric layer may comprise at least one layer of a metal oxide or semi-metal oxide. The articles may be plastic lens elements, and the sputter source may be a planar magnetron sputter source. In one embodiment of the invention the individual currents to be applied to each anode pair for each target material, in order to adjust uniformity, are determined before each coating batch by spectral measurements or otherwise on articles or witness samples coated during prior batches.
The foregoing has been provided as a convenient summary of aspects of the invention. The invention intended to be protected is, however, defined by the claims and equivalents thereof.